1. Field of the Invention
The present invention relates to a superconducting weak link device adapted to operate at the liquid nitrogen temperature or higher temperatures, and more particularly to a high temperature superconducting device which can reduce variation in the electrical characteristics thereof attributed to manufacturing process and whose operation is stable.
2. Description of the Related Art
As the materials of superconducting weak link devices of the high temperature operation type, materials such as Nb.sub.3 Ge have heretofore been used. This technique has been discussed by H. Rogalla et al in IEEE Trans. MAG-15, 536 (1985).
In addition, a superconducting device in which electrodes exhibiting superconductivity are coupled through a semiconductor or a normal-conductor has heretofore been discussed by R. B. van Dover et al in J. Appl. Phys., vol. 52, p. 7327, 1981. Besides, a three-terminal superconducting device in which the above superconducting device is additionally provided with means for changing the coupling between the electrodes exhibitive of the superconductivity on the basis of the field effect has been discussed by T. D. Clark et al in J. Appl. Phys., vol. 5, p. 2736, 1980.
In the prior-art superconducting devices mentioned above, the superconducting critical temperature (T.sub.c) of the constituent superconducting material is about 23 K. or so. Therefore, it is impossible in principle to use the devices at higher temperatures.
In the prior art, the superconducting electrodes and the semiconductor or normal-conductor are respectively made of materials composed of the individual combinations of elements. Accordingly, the superconducting device has the structure in which, on the surface of the semiconductor or the normal-conductor, the superconductor of different material is stacked and formed. On this occasion, the characteristics of the superconductor are susceptible to the surface state of the semiconductor or the normal-conductor. Therefore, the characteristics of the device having such a structure are prone to change, and it has been difficult to reproducibly fabricate the superconducting device of this type. Besides, the superconducting critical temperature (t.sub.c) of the superconductor is at most 10-20 K. or so. This signifies that the characteristics of the device are liable to become unstable due to the temperature change of the device.
Moreover, since the prior-art superconducting devices operate principally at the liquid helium temperature, they are cooled down to this temperature by a method of immersion in liquid helium or cooling with helium gas. The liquid helium, however, has posed the problems that it is very expensive and is uneconomical as a coolant and that it is, in itself, difficult of handling because of much lower temperatures than the room temperature.
These problems of the liquid helium have directly led to the problems that the superconducting devices themselves are uneconomical and are difficult of handling.